A glass plate has been generally used for a substrate for a liquid crystal display device, a substrate for an organic EL display device, a substrate for a color filter, a substrate for a solar cell, and the like. However, glass is a material that is brittle, cannot be bent, has a large specific gravity, and is unsuitable for weight reduction, for example. For that reason, studies on a plastic material as an alternative to the glass have started to be made in recent years. However, a general-purpose plastic material has a large linear expansion coefficient as compared to silver, copper, and the like, which are used for electrical circuit wiring, and hence involves problems such as peeling and breaking of wiring. In addition, heretofore, there has not been any plastic material having high transparency, weatherability, and a small birefringence simultaneously.
A silsesquioxane having a cage or partial cage structure is expected to express a specific function based on its characteristic structure, and hence has attracted attention from a variety of fields and has been studied for its application. In particular, the silsesquioxane has high transparency and heat resistance, and hence has attracted attention and has been studied as an alternative to the glass. For example, as a film substrate material used for a flat panel display, there are disclosed copolymers of a silicone resin containing a silsesquioxane having a cage structure and an acrylic resin (Patent Literatures 1 and 2). However, those copolymers each contain an acrylic resin, and hence there is no problem with mechanical strength and transparency, but weatherability is impaired in many cases. There are known film substrates each produced using a polymer containing a silsesquioxane skeleton in a main chain of the polymer without using an acrylic resin (Patent Literatures 3 and 4). Those film substrates not only have transparency but also do not undergo degradation under UV light and are excellent in weatherability, but the resultant films each have a large linear expansion coefficient. Further, an attempt at forming a film-like molded article from modified polyphenylene ether (PPE), in which a cage silsesquioxane has been incorporated, and a glass cloth has been made for the purpose of improving the fluidity of PPE. However, a processing temperature as high as 280° C. is required, and hence there is a problem with processability.